1. Field of the Invention
The present invention relates to a sewing machine and more particularly relates to a device for positioning a work pressing bar between an upper inoperative position and a lower operative position easily and smoothly with a reduced force.
2. Prior Art
Generally the work pressing bar has a work pressing foot removably attached to a lower end thereof and is positioned between an upper inoperative position where the work pressing foot is vertically spaced from a work to be stitched on a needle plate and may be changed and a lower operative position where the work pressing foot is in contact with the work to be stitched on a needle plate. The work pressing bar is normally pressed down with a pressure applied thereto by a compression spring such that the work pressing foot may press the work against the needle plate with a predetermined force while the work pressing bar is at the lower operative position. It is, therefore, required that the work pressing bar is moved to the upper inoperative position from the lower operative position against the pressing force of the compression spring by operation of a lever or the like.
The compression spring to be used is, however, of a considerably strong force because the work pressing foot is required to press a work against the needle plate with a required force, the work including various kinds of thick cloths including leathers. The machine operator is, therefore, required to exert a considerably strong force in order to move the work pressing bar to the upper inoperative position from the lower operative position by operation of a lever which is generally provided in connection with the work pressing bar.
The invention has been provided to eliminate the defects and disadvantages of the prior art. It is, therefore, a primary object of the invention to provide a device for positioning a work pressing bar, which may be easily and smoothly operated with a reduced force.
It is another object of the invention to provide a work pressing bar positioning device which is simple and compact in mechanism and reliable in operation.
It is another object of the invention to provide a work pressing bar positioning device which is operated to indirectly transmit an operating force to the work pressing bar so that the latter may be moved between an upper inoperative position and a lower operative position with a reduced force.
It is another object of the invention to provide a work pressing bar positioning device including an upper holder which is movable axially of the work pressing bar and is normally subjected to a downward spring action of a compression spring and a lower holder fixedly connected to the work pressing bar and a transmission mechanism which is operated between the upper and lower holders to transmit the downward spring action of the compression spring to the work pressing bar when the work pressing bar is in the lower operative position.
It is another object of the invention to provide a tension spring exerting a spring action for normally urging the lower holder towards the upper holder so as to move the work pressing bar to the upper inoperative position when the transmission mechanism is operated to lose the action for transmitting the downward spring action of the compression spring to the work pressing bar.
It is still another object of the invention to provide a member which is manually or automatically operated to operate the transmission mechanism.
Namely the invention substantially comprises a work pressing bar holding mechanism including an upper holder movable axially of a work pressing bar and a lower holder fixedly connected to the work pressing bar, spring means including a compression spring having a force for normally pressing down the upper holder axially of the work pressing bar, a height changing mechanism including a guide member which is operated to move between a first position laterally close to the work pressing bar and a second position laterally away from the work pressing bar and a transmission mechanism operated in response to operation of the guide member to operate the work pressing bar holding mechanism, the transmission mechanism including upper and lower links which are operatively connected to each other and to the guide member while the upper link has one end operatively connected to the upper holder and the lower link has one end operatively connected to the lower holder, such that the lower link may be operated in one direction away from the upper link against the action of a tension spring to transmit force of the compression spring to the work pressing bar by way of the upper holder, the upper and lower links and the lower holder when the guide member is in the first position and the work pressing bar is in a lower operative position. The work pressing bar is moved to an upper inoperative position by the action of the tension spring which moves the lower holder until the same comes to contact the upper holder when the guide member is in the second position.